An embodiment of the invention disclosed herein relates generally to internal combustion engines. More specifically, the invention pertains to fuel injection systems and methods that control a fuel injection event in an internal combustion engine.
Improving fuel efficiency while meeting emissions requirements is an ongoing effort in the design of internal combustion engines, including diesel engines. Typically, the reduction of emissions, such as nitrous oxide, is accompanied by increased fuel consumption. For locomotive engines in the United States, emissions compliance must be maintained over a wide range of altitudes and ambient temperatures. Accordingly, a fuel injection strategy that works well at lower altitudes may not be as desirable at high altitudes, because for example, the injection may result in emissions that fail to comply with appropriate regulations.
Traditionally, operating parameters are changed in accordance with ambient conditions such as ambient temperature and pressure, and operating conditions such as intake manifold temperature and manifold pressure, to optimize fuel efficiency. More specifically, parameters such as injection timing, engine speed and engine load have been varied in consideration of these ambient conditions. The hardware design of high-pressure unit pump fuel systems (also referred to as “unit pump systems”), for diesel internal combustion engines, limits the options available for selecting an effective strategy. The injection must occur within a defined window of the top dead center position of the piston. That is the injection must be made when injection pressure is available, which is generally within a fifty degree window around piston top dead center.
In addition, the injection pressure is fixed or predetermined for any given speed of the locomotive engine, and is not variable at a fixed speed and horsepower. In a unit pump system, a controller and solenoid flow control valve controls the flow of fuel from a low-pressure fuel reservoir into a high-pressure fuel pump and a high-pressure line, which is connected to a fuel injector. A needle valve disposed within the injector is mechanically set to open when the pressure of the high-pressure fuel line reaches a predetermined pressure. When the pressure in the high-pressure line drops below a predetermined pressure, the needle closes, thus ending injection.
A subsequent injection is not made until the pressure within the high-pressure line reaches the predetermined pressure level to open the injection valve in the fuel injector. In this type of system, the injection event is dependant directly upon the amount of pressure in the high-pressure line. Therefore, the unit pump system has control only over the timing of the injection, or when the injection is made relative to the top dead center position of the piston. Moreover, the injection pressure is the same for each given notch and cannot be independently varied for different speeds or horsepower of the locomotive. In addition, because pressure between injections must be revived, the current unit pump fuel systems used in locomotive diesel engines are limited to a single injection per injection cycle.
Other fuel systems such as the common rail fuel systems allow for more flexibility in developing fuel injection strategies. The injection event (or opening of the needle valve in the injector) is controlled by an electronic control unit (controller) and solenoid, and is not dependant on in-cycle pressure build up in the high-pressure fuel lines leading to the injector. For a common rail fuel system, the fuel supply pressure to the injector is maintained at a relatively constant, high-pressure level throughout the engine cycle. Such advanced fuel systems allow for fuel injection to take place at any time during the piston cycle and allow for multiple injections during a single cycle. In addition, the supply pressure to the injectors can be changed independent of engine speed and engine load.
Land vehicles, such as locomotives, that travel over significant distances and at varying altitudes, may experience changes to air density in the intake air manifold. Accordingly, some current fuel injection systems, including unit pump systems, consider the manifold air density in determining an injection strategy. More specifically, a locomotive controller contains a database that includes data representative of a maximum volume of fuel to be injected at predetermined engine speeds, and/or engine loads, and a predetermined manifold air pressure. Such a system is disclosed in the commonly owned U.S. Pat. No. 7,127,345.
The locomotive has sensors that detect manifold air pressure, manifold temperature and ambient barometric pressure. Based on measurements taken by these sensors a manifold air density is estimated. If the volume of fuel injected at a given engine speed and at a given manifold air pressure exceeds a predetermined volume limit, the controller adjusts the fuel demand of the locomotive to reduce horsepower. In response to the reduced horsepower, the controller alters the duration of the injection so less fuel is injected during an engine cycle. However, as noted above the unit pump systems are limited in that the injection pressure and the number of injections during an engine cycle are not variable. In addition, the calculation of the air manifold density assumes that the condition of air manifold density is similar to a density of gas within the cylinder which may not be accurate.